Transparent, oriented films made from crystallizable thermoplastics with thickness from 10 to 350 μm are well known.
In these films, there is no UV absorber present as light stabilizer and there is no flame retardant, and the films have insufficient thermoformability, the result being that neither the films nor the items and, respectively, moldings produced from them are suitable for indoor and/or outdoor applications where fire protection or, respectively, flame retardancy and thermoformability is demanded. The films do not meet the requirements for the fire tests to DIN 4102 Part 2 and Part 1 or the UL-94 test. Even after a short period in outdoor applications, these films exhibit yellowing and impairment of mechanical properties due to photooxidative degradation by sunlight.
EP-A-0 620 245 describes films whose thermal stability has been improved. These films comprise antioxidants suitable for scavenging free radicals formed within the film and degrading any peroxide formed. That specification does not propose how the UV resistance of these films might be improved. Nor does that specification state whether these films are suitable for thermoforming processes.
DE-A 2346 787 describes a flame-retardant polymer which has been phospholane-modified. Besides the polymer, its use for producing films and fibers is also claimed.
The following shortcomings become apparent when this phospholane-modified polymer is used for film production:
The polymer is very susceptible to hydrolysis and has to be very effectively predried. When the polymer is dried using dryers of the prior art it cakes, and production of a film is therefore possible only under very difficult conditions.
The films thus produced under uneconomic conditions embrittle when exposed to heat, i.e. their mechanical properties are severely impaired by the embrittlement, making the film unusable. This embrittlement occurs after as little as 48 hours of exposure to heat.
It is an object of the present invention to provide a transparent, flame-retardant, UV-resistant, thermoformable, oriented film with thickness preferably from 10 to 350 μm which is not only produced cost-effectively and has good orientability and good mechanical and optical properties but also in particular is flame-retardant, does not embrittle on exposure to heat, has good thermoformability, and has high UV resistance.
Flame retardancy means that in what is known as a fire test the transparent film meets the conditions of DIN 4102 Part 2 and in particular the conditions of DIN 4102 Part 1 and can be allocated to construction materials class B2 and in particular B1 for low-flammability materials.
It is also intended that the film pass the UL-94 test (Vertical Burning Test for Flammability of Plastic Material), permitting its grading in class 94 VTM-0. This means that 10 seconds after removal of the Bunsen burner the film has ceased to burn, after 30 seconds no smouldering is observed, and also no burning drops occur.
High UV resistance means that sunlight or other UV radiation causes no, or only extremely little, damage to the films, and that the films or molding produced from them are therefore suitable for outdoor applications and/or critical indoor applications. In particular, after a number of years in outdoor applications the films are intended not to yellow nor to exhibit any embrittlement or surface cracking, nor to have any impairment of total mechanical properties. High UV resistance therefore means that the film absorbs UV light and does not transmit light until the visible region has been reached.
Thermoformability means that the film can be thermoformed to give complex and large-surface-area moldings on commercially available thermoforming machinery without uneconomic predrying.
Good optical properties include high light transmittance (>80%), high surface gloss (>100), extremely low haze (<20%), and also low yellowness index (YI <10).
Good mechanical properties include high modulus of elasticity (EMD>3200 N/mm2; ETD>3500 N/mm2), and also good values for tensile stress at break (in MD>100 N/mm2; in TD>130 N/mm2).
Good orientability includes the capability of the film to give excellent longitudinal and transverse orientation during its production, without break-offs.
Cost-effective production includes the capability of the raw materials or raw material components needed to produce the flame-retardant film to be dried using familiar industrial dryers of the prior art. It is important that the raw materials do not cake and do not become thermally degraded. These industrial dryers of the prior art include vacuum dryers, fluidized-bed dryers; fixed-bed dryers (tower dryers).
These dryers operate at temperatures of from 100 to 170° C., at which the flame-retardant polymers mentioned cake, making film production impossible. In the vacuum dryer which provides the mildest drying conditions, the raw material traverses a range of temperatures from about 30 to 130° C. under a vacuum of 50 mbar. After this, what is known as post-drying is required, in a hopper at temperatures of from 100 to 130° C., with a residence time of from 3 to 6 hours. Even here, the polymer mentioned cakes to an extreme degree.
No embrittlement on exposure to heat means that after 100 hours of heat-conditioning at 100° C. in a circulating-air oven, the film has not embrittled and does not have disadvantageous mechanical properties.